This application is based upon Japanese Patent Application No. Hei. 11-133165 filed on May 13, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to transmit power control circuits, and particular to a transmit power control circuit suitable for a radio communication apparatus.
2. Related Art
Recently, transmit power (power level of transmit output signal) is controlled in a radio communication apparatus such as a portable telephone.
FIG. 5A shows a schematic block diagram of a transmit power control circuit according to a related art. The transmit power control circuit is basically provided with an attenuator 51 and a power amplifier 52. The attenuator 51 inputs a radio-frequency signal (RF signal) as an input signal, attenuates the input signal by attenuation depending on a control voltage from a control device such as a not-shown microcomputer, and outputs the attenuated signal as an output signal. The power amplifier 52 amplifies the output signal of the attenuator 51 with a predetermined gain, and supplies the amplified signal to an antenna as a transmit output signal. According to this transmit power control circuit, the transmit power of the transmit output signal can be controlled by changing the attenuation of the attenuator 21, as shown in FIG. 5B.
Furthermore, when the transmit power is detected to control the transmit power, as shown in FIG. 5A, a directional coupler 53 is provided at later stage of the power amplifier 52. One end of the directional coupler 53 is connected to a terminator 54, and another end is connected to a wave detector 55, so that voltage in proportion to the transmit power is measured by detecting the power by using the wave detector 55.
However, in the transmit power control circuit described in the above, since a dynamic range of the attenuation of the attenuator has a limitation, it is difficult to control the transmit power through wide range. Particularly, in a case where the CDMA (Code Division Multiple Access) method is applied to the radio communication apparatus such as the portable telephone, since the radio communication apparatus needs to control own transmit power through wide dynamic range based on a command from a base station, it is difficult to apply the above-described transmit power control circuit to the radio communication apparatus using the CDMA method.
This invention has been conceived in view of the background thus far described and its first object is to control a transmit power of a transmit power control circuit through wide dynamic range.
Its second object is to control a transmit power of a transmit power control circuit through wide dynamic range and to reduce an insertion loss.
According to the present invention, when the output signal of the variable gain circuit is outputted to the main transmission line, one end of the directional coupler is connected to a first terminator, and another end of the directional coupler is connected to a wave detector. On the contrary, when the output signal of the variable gain circuit is outputted to the auxiliary transmission line, the one end of the directional coupler is connected to the auxiliary transmission line, and said another end of the directional coupler is connected to a second terminator.
According to the above structure, a transmit power can be controlled through wide dynamic range by switching between the main transmission line and the auxiliary transmission line. Furthermore, since the directional coupler is used both for detecting the transmit power using the wave detector and for controlling the transmit power using the auxiliary transmission line, by selecting one of them, an insertion loss can be reduced.
According to another aspect of the present invention, a first switching circuit selects one of a main transmission line (A) and an auxiliary transmission line (B). An amplifying circuit, provided to the main transmission line, amplifies the output signal of the variable gain circuit, which is outputted to the main transmission line. A one end of a directional coupler is electrically connected to an auxiliary transmission line, and another end of the directional coupler is electrically connected to a terminator.
According to the above structure, a transmit power can be controlled through wide dynamic range by switching between the main transmission line and the auxiliary transmission line.